The present invention concerns a method for production of magnesium chloride with sufficiently high purity for production of magnesium metal, by dissolving magnesite (MgCO.sub.3) in hydrochloric acid and with subsequent purification of the raw solution by precipitation of undesired impurities.
Raw magnesite ore is found in many qualities according to the place of origin and with different reactivities. Macrocrystalline magnesite can have crystalites greater than 5 mm, and cryptocrystalline magnesite can have crystalites smaller than 0.01 mm. By leaching of MgCO.sub.3 the grain boundaries are first attacked, in such a way that each single crystal grain is loosened. There is therefore a great difference of reactivity between cryptocrystalline and macrocrystalline material.
It is known that magnesite has been used as a basis for production of magnesium chloride. From Hans Jedlicka: "Production of Magnesia (+99% MgO) by the Ruthner-HCl-Route", Andritz-Ruthner Industrianlagen Aktiengesellschaft, Aichholzgasse 51-53, A-1120 Vienna, pp 5-7, there is a known method for production of magnesium oxide based on leaching of magnesite with hydrochloric acid. Magnesium, as well as iron, aluminium, chromium, manganese, calcium etc. in the raw material are dissolved by the formation of chlorides.
In this process finely ground magnesite is used. The process is based on a raw material having a grain size smaller than 0.3 mm. In addition to a resource demanding and expensive grinding process, foaming problems often arise when such a finely ground material is used. Fine grains have a tendency to stick to the surface of the bubbles and form a stable foam which makes the separation of the gas from the fluid difficult. This gives low production per volume of the reactor.
According to this process possible remaining hydrochloric acid is neutralized by adding ultra-basic reactive flue-dust until a pH-value between 4 and 6 is reached. It is stated that by this pH value hydroxides of all trivalent impurities are completely precipitated. Our experience is, however, that within this pH range the precipitation of divalent heavy metals, e.g. nickel, will be insufficient.
From U.S. Pat. No. 3,980,753 there is known a process for production of magnesia of very high purity (98-99%). The brine therefore must be low in Ca and alkali metals. The magnesia is produced from a magnesite waste material which can be raw magnesite, brucite, dolomite or other magnesium containing ores. The material is dissolved in hydrochloric acid of 15-32% concentration. Impurities are precipitated by adjusting the pH to 4-9 and the purified magnesium chloride solution is thermally decomposed into magnesia and hydrochloric gas.
For removal of undesired impurities as phosphates and heavy metals, the solution is oxidized and the pH is raised to 4-9 (6-8 preferred) to precipitate the contaminants mainly as hydroxides. This pH will only bring the content of Ni down to a few (2-3) mg/kg liquid, which will be insufficient for the production of high quality magnesium metal.
As some of the raw materials used are high in calcium, the process comprises addition of sulphates in the following purification steps to remove calcium as calcium sulphate. This procedure will render a brine far too high in sulphur to be acceptable for metal production.
The process according to Jedlicka and to U.S. Pat. No. 3,980,753 are described as batch processes using a stirred reactor for leaching of the raw material. It is preferred to use a particle size less than respectively 0.3 mm and 3 mm for the raw material. When dissolving finely ground magnesite in hydrochloric acid, it should be evident that the particles must be suspended in the liquid to obtain reasonable reaction rates. Because of the mixing, a stirred reactor will operate at a concentration of unreacted HCl equal to the outgoing liquid. This concentration should be kept moderately low to avoid substantial losses of HCl by the leaving gas.
The capacity of a stirred reactor is limited by its ability to remove gas bubbles from the liquid phase. It should be evident that the reactor will overflow if the superficial gas velocity exceeds the rising velocity of the bubbles in the liquid phase. Thus the production load must be kept well below this limit. According to Perry, Chemical Engineer's Handbook, 5th edition, p. 18-71, fig.18-117 the rising velocity of bubbles in a stirred reactor will at maximum be in the order of 0.06 m/s.